Modulated microwave signals are used to carry information in a wide variety of electronic communications systems. Examples include modulated microwave signals used to transmit voice and data or video signals from a ground transmitter through space to a satellite, and then back from the satellite to a ground receiver. Another example is a television transmitter, which transmits modulated signals that carry the picture and sound to television sets. Another example is a cellular base station, which transmits modulated microwave signals that carry the voice information to cellular phones. Such signals must be accurately measured for conformance to systems specifications and for accurate modeling of deviations from ideal performance.
It is also desirable to use such modulated microwave signals to characterize nonlinear devices, such as power amplifiers, used in communications systems, because these are the signals that the devices receive in operation. Nonlinear electronic devices are often the most difficult elements to model accurately in communications simulations. A recent example is the design and simulation of power amplifiers for use in digital cellular applications. In this case, the transmit power amplifier must be operated at or near saturation for high efficiency, and still meet stringent adjacent channel power requirements. This is an example where accurate, computationally efficient nonlinear models are required to make the proper design tradeoffs. Also known as black-box models, these models are computationally efficient because they transform an input waveform to the correct output waveform without resorting to the details of circuit operation. These models seek to characterize the nonlinear amplifier through the use of a selected set of probing signals. The degree of predictive fidelity of these simulation models must be checked with the class of operational signals expected, such as modulated microwave signals.
A prior procedure of measurement of modulated microwave signals is to record directly the radio frequency (RF) or microwave signal by means of a digital storage oscilloscope or other waveform recorder. For measurement of nonlinear devices, the RF or microwave frequency waveforms must be measured at the input and output of the device to find the input to output characteristic. These waveforms should be recorded at a number of input power levels throughout the operating range of the nonlinear device. Examples of such nonlinear components are a solid-state power amplifier or a traveling-wave tube amplifier. Time-domain instrumentation can record the waveform data digitally and store the data directly on a controlling computer.
Typical instruments used for recording waveform data are a digital storage oscilloscope (DSO), a microwave transition analyzer (MTA), or a recently developed nonlinear vector network analyzer (NVNA). The accuracy of these instruments for measuring high frequency signals is limited by linear amplitude and phase distortion. The MTA and high bandwidth DSO have significant phase and amplitude distortion beginning at frequencies above about 15.0 GHz. The NVNA is based on an MTA, but the NVNA comes with calibration standards and an extensive calibration routine so that phase and amplitude distortion, and other errors, are analytically removed from the measurements. The Hewlett-Packard NVNA performs calibrated time-domain measurements of signals up to 50.0 GHz. This NVNA includes calibration standards and software that calibrates the sampling oscilloscope for analog and digital nonlinearity, and gain and phase responses over a frequency range. Hence, the NVNA can provide accurate waveforms up to 50.0 GHz. The NVNA calibration eliminates any inaccuracy associated with the gain and phase response of the sampling oscilloscope, however, the NVNA still has limitations imposed by a limited number of samples and phase noise errors. The NVNA is a very complex and expensive system, however, and the calibration process is cumbersome. Additionally, the NVNA cannot be used to characterize a modulator providing an arbitrary microwave waveform.
Another prior waveform measurement approach is to use an uncalibrated downconverter with separate I and Q output signals. These I and Q output signals are then recorded by means of a DSO. This technique also yields the time-domain baseband waveform, but is corrupted by the undesired distortions of the downconverter, including the I/Q imbalance between the I and Q signals. The unknown distortions are usually large enough to render the waveform data useless for any precision application such as communications system modeling.
A recent measurement approach measures the transmission response of a frequency translating device (FTD), such as a mixer, as disclosed in the related patent. The response of the FTD, including a downconverter, may be measured by means of the baseband-double-sideband-mixer FTD characterization method, as described in the related patent. In this FTD characterization method, three pair wise combinations of an upconverter referred to herein as a transmitter, a test mixer, and the downconverter referred to herein as a receiver, are measured. The transmission response of the downconverting receiver is then calculated from these measurements. The test configuration setup for this FTD characterization method consists of connecting an upconverting transmitter FTD to a downconverting receiver FTD with both using the same local oscillator (LO) but with a phase shifter in the downconverter LO path. A vector network analyzer (VNA) is used to measure this first paired combination at two relative LO phase settings 90.degree. apart. The additional test mixer is used in the second of these measurements as an upconverter and in the third of these measurements as a downconverter. The FTD characterization method requires that the test mixer have the same frequency response, that is a reciprocal frequency response, whether the test mixer is used as an upconverter or a downconverter. In practice, commonly available double-balanced mixers exhibit this reciprocal response if a low voltage standing wave ratio (VSWR) is provided on all ports by use of fixed attenuators. These six measurements, for the three configurations with zero and with ninety degree phase shift, are sufficient to extract the frequency response of all three FTDs, including the downconverting receiver. By mathematically combining the six measurements provided by the three-setup configuration, with and without the 90.degree. phase shift, the lowpass equivalent (LPE) frequency response of the downconverting receiver may be obtained. This three-pair FTD characterization method has not been applied to the characterization of an arbitrary microwave signal. These and other disadvantages are solved or reduced using the invention.